A quantum reaction dynamics study of the translational, vibrational, and rotational motion effects on the HD + H3+ reaction

Time-dependent, quantum reaction dynamics wavepacket approach is employed to investigate the impacts of the translational, vibrational, and rotational motion on the HD+H(3)(+) → H(2)D(+) + H(2) reaction using the Xie-Braams-Bowman potential energy surface [Z. Xie, B. J. Braams, and J. M. Bowman, J. Chem. Phys. 122, 224307 (2005)]. We treat this five atom reaction with a seven-degree-of-freedom model by fixing one Jacobi and one torsion angle related to H(3) (+) at the lowest saddle point geometry of the potential energy surface. The initial state selected reaction probabilities show that the rotational excitations of H(+)-H(2) greatly enhance the reactivity with the reaction probabilities increased double at high rotational states compared to the ground state. However, the vibrational excitations of H(3) (+) hinder the reactivity. The ground state reaction probability shows no reaction threshold for this exoergic reaction, and as the translational energy increases, the reaction probability decreases. Furthermore, reactive resonances and zero point energy play very important roles on the reaction dynamics. The obtained integral cross section has the character of an exoergic reaction without a threshold: it decreases with the translational energy increasing. The calculated thermal rate constants using this seven-degree-of-freedom model are in agreement with a later experiment measurement.     

Influence of nuclear exchange on nonadiabatic electron processes in H(+)+H2 collisions

H(+)+H(2) collisions are studied by means of a semiclassical approach that explicitly accounts for nuclear rearrangement channels in nonadiabatic electron processes. A set of classical trajectories is used to describe the nuclear motion, while the electronic degrees of freedom are treated quantum mechanically in terms of a three-state expansion of the collision wavefunction. We describe electron capture and vibrational excitation, which can also involve nuclear exchange and dissociation, in the E = 2-1000 eV impact energy range. We compare dynamical results obtained with two parametrizations of the potential energy surface of H(3)(+) ground electronic state. Total cross sections for E > 10 eV agree with previous results using a vibronic close-coupling expansion, and with experimental data for E < 10 eV. Additionally, some prototypical features of both nuclear and electron dynamics at low E are discussed.     

Highly accurate quartic force fields, vibrational frequencies, and spectroscopic constants for cyclic and linear C3H3(+)

High levels of theory have been used to compute quartic force fields (QFFs) for the cyclic and linear forms of the C(3)H(3)(+) molecular cation, referred to as c-C(3)H(3)(+) and l-C(3)H(3)(+). Specifically, the singles and doubles coupled-cluster method that includes a perturbational estimate of connected triple excitations, CCSD(T), has been used in conjunction with extrapolation to the one-particle basis set limit, and corrections for scalar relativity and core correlation have been included. The QFFs have been used to compute highly accurate fundamental vibrational frequencies and other spectroscopic constants by use of both vibrational second-order perturbation theory and variational methods to solve the nuclear Schro?dinger equation. Agreement between our best computed fundamental vibrational frequencies and recent infrared photodissociation experiments is reasonable for most bands, but there are a few exceptions. Possible sources for the discrepancies are discussed. We determine the energy difference between the cyclic and linear forms of C(3)H(3)(+), obtaining 27.9 kcal/mol at 0 K, which should be the most reliable available. It is expected that the fundamental vibrational frequencies and spectroscopic constants presented here for c-C(3)H(3)(+) and l-C(3)H(3)(+) are the most reliable available for the free gas-phase species, and it is hoped that these will be useful in the assignment of future high-resolution laboratory experiments or astronomical observations.     

Communication: state-to-state quantum dynamics study of the OH + CO → H + CO2 reaction in full dimensions (J = 0)

A full dimensional state-to-state quantum dynamics study is carried out for the prototypical complex-formation OH + CO → H + CO(2) reaction in the ground rovibrational initial state on the Lakin-Troya-Schatz-Harding potential energy surface by using the reactant-product decoupling method. With three heavy atoms and deep wells on the reaction path, the reaction represents a huge challenge for accurate quantum dynamics study. This state-to-state calculation is the first such a study on a four-atom reaction other than the H(2) + OH ? H(2)O + H and its isotope analogies. The product CO(2) vibrational and rotational state distributions, and product energy partitioning information are presented for ground initial rovibrational state with the total angular momentum J = 0.     

Mechanistic study of the electrochemical oxygen reduction reaction on Pt(111) using density functional theory

Density functional theory (DFT) was used to study the electrolyte solution effects on the oxygen reduction reaction (ORR) on Pt(111). To model the acid electrolyte, an H(5)O(2)(+) cluster was used. The vibrational proton oscillation modes for adsorbed H(5)O(2)(+) computed at 1711 and 1010 cm(-1), in addition to OH stretching and H(2)O scissoring modes, agree with experimental vibrational spectra for proton formation on Pt surfaces in ultrahigh vacuum. Using the H(5)O(2)(+) model, protonation of adsorbed species was found to be facile and consistent with the activation barrier of proton transfer in solution. After protonation, OOH dissociates with an activation barrier of 0.22 eV, similar to the barrier for O(2) dissociation. Comparison of the two pathways suggests that O(2) protonation precedes dissociation in the oxygen reduction reaction. Additionally, an OH diffusion step following O protonation inhibits the reaction, which may lead to accumulation of oxygen on the electrode surface.     

Primary branching ratios for the low-temperature reaction of state-prepared N2+ with CH4, C2H2, and C2H4

Product branching ratios (BRs) are reported for ion-molecule reactions of state-prepared nitrogen cation (N(2)(+)) with methane (CH(4)), acetylene (C(2)H(2)). and ethylene (C(2)H(4)) at low temperature using a modified ion imaging apparatus. These reactions are performed in a supersonic nozzle expansion characterized by a rotational temperature of 40 ± 5K. For the N(2)(+) + CH(4) reaction, a BR of 0.83:0.17 is obtained for the dissociative charge-transfer (CT) reaction that gives rise to the formation of CH(3)(+) and CH(2)(+) product ions, respectively. The N(2)(+) + C(2)H(2) ion-molecule reaction proceeds through a nondissociative CT process that results in the sole formation of C(2)H(2)(+) product ions. The reaction of N(2)(+) with C(2)H(4) leads to the formation of C(2)H(3)(+) and C(2)H(2)(+) product ions with a BR of 0.74:0.26, respectively. The reported BR for the N(2)(+) + C(2)H(4) reaction is supportive of a nonresonant dissociative CT mechanism similar to the one that accompanies the N(2)(+) + CH(4) reaction. No dependence of the branching ratios on N(2)(+) rotational level was observed. In addition to providing direct insight into the dynamics of the state-prepared N(2)(+) ion-molecule reactions with the target neutral hydrocarbon molecules, the reported low-temperature BRs are also important for accurate modeling of the nitrogen-dominated upper atmosphere of Saturn's moon, Titan.     

Multipodal coordination of a tetracarboxylic crown ether with NH4(+): a vibrational spectroscopy and computational study

The elucidation of the structural requirements for molecular recognition by the crown ether (18-crown-6)-2,3,11,12-tetracarboxylic acid (18c6H(4)) and its cationic complexes constitutes a topic of current fundamental and practical interest in catalysis and analytical sciences. The flexibility of the central ether ring and its four carboxyl side arms poses important challenges to experimental and theoretical approaches. In this study, infrared action vibrational spectroscopy and quantum mechanical computations are employed to characterize the conformational structure of the isolated gas phase complex formed by the 18c6H(4) host with NH(4)(+) as guest. The results show that the most stable gas-phase structure is a barrel-like conformation sustained by tetrapodal H-bonding of the ammonia cation with two C=O side groups and with four oxygen atoms of the ether ring in a bifurcated arrangement. Interestingly, a similar structure had been proposed in previous crystallographic studies. The experiment also provides evidence for a significant contribution of a higher energy bowl-like conformer with features resembling those adopted by 18c6H(4) in the analogous complexes with secondary amines. Such a conformation displays H-bonding between confronted side carboxyl groups and tetrapodal binding of the NH(4)(+) with the ether ring and with one C=O group. Structures involving even more extensive intramolecular H-bonding in the 18c6H(4) substrate are found to lie higher in energy and are ruled out by the experiment.     

Nuclear spin dependence of the reaction of H(3)+ with H2. I. Kinetics and modeling

The chemical reaction H(3)(+) + H(2) → H(2) + H(3)(+) is the simplest bimolecular reaction involving a polyatomic, yet is complex enough that exact quantum mechanical calculations to adequately model its dynamics are still unfeasible. In particular, the branching fractions for the "identity," "proton hop," and "hydrogen exchange" reaction pathways are unknown, and to date, experimental measurements of this process have been limited. In this work, the nuclear-spin-dependent steady-state kinetics of the H(3)(+) + H(2) reaction is examined in detail, and employed to generate models of the ortho:para ratio of H(3)(+) formed in plasmas of varying ortho:para H(2) ratios. One model is based entirely on nuclear spin statistics, and is appropriate for temperatures high enough to populate a large number of H(3)(+) rotational states. Efforts are made to include the influence of three-body collisions in this model by deriving nuclear spin product branching fractions for the H(5)(+) + H(2) reaction. Another model, based on rate coefficients calculated using a microcanonical statistical approach, is appropriate for lower-temperature plasmas in which energetic considerations begin to compete with the nuclear spin branching fractions. These models serve as a theoretical framework for interpreting the results of laboratory studies on the reaction of H(3)(+) with H(2).     

A full dimensional, nine-degree-of-freedom, time-dependent quantum dynamics study for the H2+C2H reaction

A full dimensional, nine-degree-of-freedom (9DOF), time-dependent quantum dynamics wave packet approach is presented for the study of the H2+C2H-->H+C2H2 reaction system. This is the first full dimensional quantum dynamics study for a diatom-triatom reaction system. The effects of the initial vibrational and rotational excitations of the reactants on the reactivity of this reaction are investigated. This study shows that vibrational excitations of H2 enhance the reactivity; whereas, the vibrational excitations of C2H only have a small effect on the reaction probability. In addition, the bending excitations of C2H, compared to the ground state reaction probability, hinder the reactivity. Comparison of the ground state reaction probabilities of the 9DOF and 8DOF shows the reaction probability from the full dimensional calculation is larger, with more prominent resonance features.     

An ab initio based global potential energy surface describing CH5+ --> CH3+ + H2

A full-dimensional, ab initio based potential energy surface (PES) for CH(5)(+), which can describe dissociation is reported. The PES is a precise fit to 36173 coupled-cluster [CCSD(T)] calculations of electronic energies done using an aug-cc-pVTZ basis. The fit uses a polynomial basis that is invariant with respect to permutation of the five H atoms, and thus describes all 120 equivalent minima. The rms fitting error is 78.1 cm(-1) for the entire data set of energies up to 30,000 cm(-1) and a normal-mode analysis of CH(5)(+) also verifies the accuracy of the fit. Two saddle points have been located on the surface as well and compared with previous theoretical work. The PES dissociates correctly to the fragments CH(3)(+) + H(2) and the equilibrium geometry and normal-mode analyses of these fragments are also presented. Diffusion Monte Carlo calculations are done for the zero-point energies of CH(5)(+) (and some isotopologs) as well as for the separated fragments of CH(5)(+), CH(3)(+) + H(2) and those of CH(4)D(+), CH(3)(+) + HD and CH(2)D(+) + H(2). Values of D(0) are reported for these dissociations. A molecular dynamics calculation of CH(4)D(+) dissociation at one total energy is also performed to both validate the applicability of the PES for dynamics studies as well as to test a simple classical statistical prediction of the branching ratio of the dissociation products.     

Geometric phase effects in resonance-mediated scattering: H + H2(+) on its lowest triplet electronic state

We report a quantum dynamics study of H + H(2)(+) (v(0) = 0, j(0) = 0) scattering on its lowest triplet state, for J = 0 total angular momentum and total energies up to 1.85 eV. This provides a benchmark example of indirect resonance-mediated reaction in presence of a conical intersection (CI). Visualization of time-dependent wave packets shows significant "looping" around the CI, which is facilitated by long-lived H(3)(+) scattering resonances, predominant at low energies. State-to-state inelastic transition probabilities exhibit a highly oscillatory structure and pronounced geometric phase effects, which, in contrast to direct reactions, are more strongly marked at lower energies.     

Communication: A concerted mechanism between proton transfer of Zundel anion and displacement of counter cation

Ab initio path integral molecular dynamics simulation of M(+)(H(3)O(2)(-)) (M = Li, Na, and K) has been carried out to analyze how the structure and dynamics of a low-barrier hydrogen-bonded Zundel anion, H(3)O(2)(-), can be affected by the counter alkali metal cation, M(+). Our simulation predicts that the quantum proton transfer in Zundel anion can be strongly coupled to the motion of counter cation located nearby. A smaller cation can induce larger structural distortion of the Zundel anion fragment making the proton transfer barrier higher, and hence, lower the vibrational excitation energy. It is also argued that a large H∕D isotope effect is present.     

A rigorous full-dimensional quantum dynamics calculation of the vibrational energies of H3O-2

The vibrational energy levels of the H(3)O-(2) anion have been calculated using a rigorous quantum dynamics method based on an accurate ab initio potential energy surface. The eigenvalue problem is solved using the two-layer Lanczos iterative diagonalization algorithm in a mixed grid/nondirect product basis set, where the system Hamiltonian is expressed in a set of orthogonal polyspherical coordinates. The lowest 312 vibrational energy levels in each inversion symmetry, together with a comparison of fundamental frequencies with previous quantum dynamics calculations, are reported. Finally, a statistical analysis of nearest level spacing distribution is carried out, revealing a strongly chaotic nature.     

Reaction of C2H2(+) (n · ν2, m · ν5) with NO2: reaction on the singlet and triplet surfaces

Integral cross sections and product recoil velocity distributions were measured for reaction of C(2)H(2)(+) with NO(2), in which the C(2)H(2)(+) reactant was prepared in its ground state, and with mode-selective excitation in the cis-bend (2ν(5)) and CC stretch (n · ν(2), n = 1, 2). Because both reactants have one unpaired electron, collisions can occur with either singlet or triplet coupling of these unpaired electrons, and the contributions are separated based on distinct recoil dynamics. For singlet coupling, reaction efficiency is near unity, with significant branching to charge transfer (NO(2)(+)), O(-) transfer (NO(+)), and O transfer (C(2)H(2)O(+)) products. For triplet coupling, reaction efficiency varies between 13% and 19%, depending on collision energy. The only significant triplet channel is NO(+) + triplet ketene, generated predominantly by O(-) transfer, with a possible contribution from dissociative charge transfer at high collision energies. NO(2)(+) formation (charge transfer) can only occur on the singlet surface, and appears to be mediated by a weakly bound complex at low energies. O transfer (C(2)H(2)O(+)) also appears to be dominated by reaction on the singlet surface, but is quite inefficient, suggesting a bottleneck limiting coupling to this product from the singlet reaction coordinate. The dominant channel is O(-) transfer, producing NO(+), with roughly equal contributions from reaction on singlet and triplet surfaces. The effects of C(2)H(2)(+) vibration are modest, but mode specific. For all three product channels (i.e., charge, O(-), and O transfer), excitation of the CC stretch fundamental (ν(2)) has little effect, 2 · ν(2) excitation results in ～50% reduction in reactivity, and excitation of the cis-bend overtone (2 · ν(5)) results in ～50% enhancement. The fact that all channels have similar mode dependence suggests that the rate-limiting step, where vibrational excitation has its effect, is early on the reaction coordinate, and branching to the individual product channels occurs later.     

Crystal structure of ClF(4)(+)SbF(6)(-), normal coordinate analyses of ClF(4)(+), BrF(4)(+), IF(4)(+), SF(4), SeF(4), and TeF(4), and simple method for calculating the effects of fluorine bridging on the structure and vibrational spectra of ions in a strongly interacting ionic solid.

The crystal structure of the 1:1 adduct ClF(5).SbF(5) was determined and contains discrete ClF(4)(+) and SbF(6)(-) ions. The ClF(4)(+) cation has a pseudotrigonal bipyramidal structure with two longer and more ionic axial bonds and two shorter and more covalent equatorial bonds. The third equatorial position is occupied by a sterically active free valence electron pair of chlorine. The coordination about the chlorine atom is completed by two longer fluorine contacts in the equatorial plane, resulting in the formation of infinite zigzag chains of alternating ClF(4)(+) and cis-fluorine bridged SbF(6)(-) ions. Electronic structure calculations were carried out for the isoelectronic series ClF(4)(+), BrF(4)(+), IF(4)(+) and SF(4), SeF(4), TeF(4) at the B3LYP, MP2, and CCSD(T) levels of theory and used to revise the previous vibrational assignments and force fields. The discrepancies between the vibrational spectra observed for ClF(4)(+) in ClF(4)(+)SbF(6)(-) and those calculated for free ClF(4)(+) are largely due to the fluorine bridging that compresses the equatorial F-Cl-F bond angle and increases the barrier toward equatorial-axial fluorine exchange by the Berry mechanism. A computationally simple model, involving ClF(4)(+) and two fluorine-bridged HF molecules at a fixed distance as additional equatorial ligands, was used to simulate the bridging in the infinite chain structure and greatly improved the fit between observed and calculated spectra.     

Kinetics and dynamics of the NH3 + H → NH2 + H2 reaction using transition state methods, quasi-classical trajectories, and quantum-mechanical scattering

On a recent analytical potential energy surface developed by two of the authors, an exhaustive kinetics study, using variational transition state theory with multidimensional tunneling effect, and dynamics study, using both quasi-classical trajectory and full-dimensional quantum scattering methods, was carried out to understand the reactivity of the NH(3) + H → NH(2) + H(2) gas-phase reaction. Initial state-selected time-dependent wave packet calculations using a full-dimensional model were performed, where the total reaction probabilities were calculated for the initial ground vibrational state and for four excited vibrational states of ammonia. Thermal rate constants were calculated for the temperature range 200-2000 K using the three methods and compared with available experimental data. We found that (a) the total reaction probabilities are very small, (b) the symmetric and asymmetric N-H stretch excitations enhance the reactivity, (c) the quantum-mechanical calculated thermal rate constants are about one order of magnitude smaller than the transition state theory results, which reproduce the experimental evidence, and (d) quasi-classical trajectory calculations, which were performed with the main goal of analyzing the influence of the zero-point energy problem on the final dynamics results, reproduce the quantum scattering calculations on the same surface.     

Multiphoton ion pair spectroscopy (MPIPS) with ultrashort laser pulses for the H2 molecule

Time-dependent Schr?dinger equation, TDSE, simulations have been performed in order to prepare and study via MPIPS the evolution of vibrational wave packets on the ion pair electronic state potentials B'B1Sigma(u)(+) and Hh1Sigma(g)(+) of the H2 molecule. Using ab initio potential surfaces and transition moments, we present two- and three-photon excitation schemes with ultrashort pulses (tau 相似文献   

Ab initio centroid path integral molecular dynamics: application to vibrational dynamics of diatomic molecular systems

An ab initio centroid molecular dynamics (CMD) method is developed by combining the CMD method with the ab initio molecular orbital method. The ab initio CMD method is applied to vibrational dynamics of diatomic molecules, H2 and HF. For the H2 molecule, the temperature dependence of the peak frequency of the vibrational spectral density is investigated. The results are compared with those obtained by the ab initio classical molecular dynamics method and exact quantum mechanical treatment. It is shown that the vibrational frequency obtained from the ab initio CMD approaches the exact first excitation frequency as the temperature lowers. For the HF molecule, the position autocorrelation function is also analyzed in detail. The present CMD method is shown to well reproduce the exact quantum result for the information on the vibrational properties of the system.     

Evolution of structure in CH5 + and its deuterated analogues

Diffusion Monte Carlo simulations are used to investigate the effects of deuteration on the fluxionality of CH(5)(+) or CD(5)(+), using an ab initio potential surface, developed by Jin, Braams, and Bowman [J. Phys. Chem. 2006, 110, 1569]. We find that partial deuteration quenches the fluxional behavior. The spectral consequences are also investigated. We find that, while CH(5)(+) and CD(5)(+) are nearly spherical tops, partial deuteration breaks the rotational symmetry and the mixed isotopologues are generally better characterized as symmetric tops. In addition, we investigate the effects of deuteration on the low-resolution vibrational spectrum and anticipate that signatures of this delocalization will be observable in the vibrational spectrum.